Figure 124

In this autosomal recessive disorder the apical transport of cystine and the dibasic amino acids is defective. Differences in the urinary excretion of cystine in obligate heterozygotes and intestinal amino acid transport studies in homozygotes have provided the basis for defining three distinct phenotypes of cystin-uria [9]. Genetic studies have identified mutations in the gene (SCL3A1) encoding a high-affinity transporter for cystine and the dibasic amino acids in patients with type I cystinuria [10,11]. In patients with type III cystinuria, SCL3A1 was excluded as the disease-causing gene [12]. A second cystin-uria-susceptibility gene recently has been mapped to chromosome 19 [13].